1. Field of the Invention
The present invention relates to circuits for operating light-emitting diodes.
2. Related Technology
Even though in the present description the invention will be described with reference to emergency-lighting devices with LEDs, it is to be understood that the invention relates quite generally to LED-operating circuits.
Emergency-lighting devices accordingly exhibit by way of central element an energy storage unit—in particular, a battery or an accumulator—which during normal operation or charging operation is charged by the general mains supply voltage. For this purpose a charging circuit is provided which is connected to the mains supply voltage on the input side and permanently supplies energy during charging operation of the energy storage unit, which stores this energy. Only in the case where an emergency condition arises—which is ordinarily detected autonomously by devices of such a type through a monitoring of the mains supply voltage—is a change-over made to emergency-light operation, in which the light-source is activated and operated, to which end—where required—the energy made available by the energy storage unit is utilized. Since the storage capacity of the energy storage unit is of course limited, light-sources are preferably employed that consume relatively little energy. Accordingly, emergency-lighting devices of such a type are preferably equipped with gas-discharge lamps, in particular fluorescent tubes. However, light-sources in the form of light-emitting semiconductors, in particular LEDs, are also increasingly finding application, since these light-sources also exhibit a high efficiency and can accordingly be employed in energy-saving manner.